1. Field of the Invention
The invention relates to a fixing apparatus and a fixing method for fixing a developing agent image on a recording medium by heating and pressing by a heated endless fixing belt.
2. Description of the Related Art
Recently, as a fixing apparatus which is used for image forming devices such as electrophotographic copy machines and printers to fix a toner image to a sheet of paper by heating and pressing, there is developed a belt type fixing apparatus which heats an endless belt type fixing belt and passes the sheet of paper through a nip portion between the fixing belt and a pressing roller which is in pressing contact with the fixing belt to fix the toner image by heating and pressing because a fixing temperature has high uniformity and high temperature resumability and a good fixing property can be obtained for an image which has toners overlaid in the same manner as a full-color image as compared with a roller fixing type fixing apparatus which passes a sheet of paper between a pair of fixing rollers to fix a toner image by heating and pressing.
Conventionally, according to the belt-type fixing apparatus, a fixing belt 4 was put around a heating roller 2 having for example a heater lamp 1 of 700 W at the rotation center therein and a support roller 3 to heat the fixing belt 4 in contact with the heating roller as shown in FIG. 1, a pressure roller 6 was contacted under pressure to a position facing the support roller 3 through the fixing belt 4, and a sheet of paper 7 was passed through a nip section formed by the fixing belt 4 heated to a fixable temperature by the heating roller 2 and the pressure roller 6 so to fix a toner image by heating and pressing.
And, Japanese Patent Application Laid-Open Publication No. 11-344885 discloses that a heating roller has therein heater lamps having larger and smaller sizes to heat a fixing belt. Such conventional heater lamps with larger and smaller sizes are designed to be switched from the larger heater lamp to the smaller heater lamp to prevent the surface temperature of a none-paper passing section of the heating roller from rising excessively when small size sheets of paper are successively conveyed, thereby providing a long life and safety of the heating roller.
Generally, the fixing apparatus needs a high-temperature heating source and its electric power consumption is about 60 to 70% of a total electric power consumption of the image forming device. Therefore, the belt type fixing apparatus is required to save the electric power consumption at the fixing operation without degradation of speed-up in response to the demand for the speed-up and the saving of the electric power consumption of the image forming device lately. The aforesaid fixing apparatus, however, is not designed to achieve the saving of the electric power consumption.
As one means to realize the saving of the electric power consumption of the fixing apparatus, it is effective to reduce the power consumption of the heating source at the fixing operation. At the same time, the power requirements of the fixing apparatus can be reduced by shortening a warm-up time for raising the fixing belt to a predetermined temperature after turning on the image forming device or by shortening the first copying time from a stand-by state for the next fixing operation after completing the fixing operation to a temperature increase of the fixing belt to a fusable state.
But, the heat source of the fixing apparatus is generally required to be set to have high output so that a good fixing property can be obtained even for a developed image such as a full-color image having a large amount of toner used for overlaying multicolor. Therefore, the same high-output heating source must be used for even a developed image with a little amount of toner such as a monochrome text image, resulting in consuming an excessive power for the developed image with a small amount of toner.
Besides, the heating source is always under ON/OFF control in order to keep the fixing temperature in a predetermined range during the fixing operation, but the temperature of the fixing belt involved in the ON/OFF operation of the heating source becomes considerably uneven when the heating source has high output, and the fixed image is caused to have an uneven gloss due to the uneven temperature of the fixing belt. Meanwhile, it is desired recently that the uneven gloss caused by the uneven temperature is reduced as low as possible when a full-color image is obtained from image data of digital photographs or the like, particularly when a high quality image is demanded.
Besides, when the heating source is determined to have high output in order to reduce the warm-up time or the first copy time, the high-output heating source must be ON/OFF controlled even in the stand-by state after warming up so to keep the stand-by state, resulting in increasing the power consumption in the stand-by state.
Therefore, the belt type fixing apparatus is desired to provide a good fixing property regardless of an amount of toner of the developed image while realizing the effective power saving and to improve the indication quality of the fixed image. Furthermore, it is desired that the power consumption is also reduced in the stand-by state when the warm-up time and the first copy time are reduced in order to provide the effective power reduction.
It is an object of the invention to reduce the electric power consumption of a fixing belt type fixing apparatus, which can finely deal with various developing images, during a fixing operation without lowering the indication quality of a fixed image.
It is also an object of the invention to reduce the power consumption even in the stand-by state when the warm-up time and the first copy time are reduced to speed up the fixing time and the power consumption at warming up and first copying is reduced in the fixing belt type fixing apparatus.
The invention provides a fixing apparatus, which comprises first heating means, which form an endless belt shape, for heating a recording medium having a developing agent image thereon; pressing means, which form a roller shape, for fixing the developing agent image by catching and conveying the recording medium in cooperation with the first heating means; second heating means, which form a roller shape and come into contact with the first heating means, for heating the first heating means to a predetermined temperature; a plurality of heating means, which are disposed in the second heating means and have different output, for heating the second heating means entirely in the longitudinal direction; and control means for controlling the plurality of heating means.
The invention also provides a fixing apparatus which comprises a fixing belt, which forms an endless belt shape, to configured to heat a recording medium having a developing agent image thereon; a pressing roller, which forms a roller shape, configured to fix the developing agent image by catching and conveying the recording medium in cooperation with the fixing belt; a heating roller, which forms a roller shape and comes into contact with the fixing belt, configured to heat the fixing belt to a predetermined temperature; a plurality of heater lamps, which are disposed in the heating roller and have different output, to heat the heating roller entirely in the longitudinal direction; and a control device configured to control the plurality of heater lamps.
The invention also provides a fixing method for fixing a developing agent image by heating and pressing by conveying a recording medium having the developing agent image while holding it between a fixing belt heated in contact with a heating roller and a pressing roller, comprising the steps of heating the heating roller by selectively controlling to turn ON/OFF a plurality of heater lamps which are disposed in the heating roller, have different output and can heat the heating roller entirely in its longitudinal direction; heating the fixing belt by contacting the heating roller to the fixing belt; and passing the recording medium through a nip section formed between the fixing belt which is separated from the heating roller and the pressing roller which is in pressing contact with the fixing belt.